It has been estimated that up to 80% of human cancers result from DNA damage due to environmental exposures. The molecular mechanisms for repairing such DNA lesions and the cellular processes which result in this damage causing neoplastic transformation are incompletely understood. Cellular replication is probably a critical step in the propagation of the mutagenic lesions which result in cellular transformation. If a cell is able to repair a (spontaneous or induced) genetic abnormality prior to DNA synthesis, malignant transformation may be avoided. The experiments in this grant application are designed to investigate the biochemical mechanisms by which cells delay new DNA synthesis until DNA damage has been repaired. Our preliminary data suggests that the "tumor suppressor" gene, p53, contributes to this process: 1) Certain types of DNA damage result in increased levels of p53 protein (by a post-transcriptional mechanism) in temporal association with transient inhibition of DNA synthesis; and 2) cells with normal, but not abnormal, p53 genes, exhibit transient G, arrest following certain types of DNA damage. A significant role for p53 protein in the cellular response to DNA damage would be consistent with the observations of high frequencies of p53 gene abnormalities in a wide variety of human tumors. Cell cycle alterations in response to DNA damage will be characterized in cells with normal and altered (in some instances by genetic manipulation) p53 genes. In addition, variations in the cell cycle changes and p53 protein alterations will be evaluated following different types of DNA damage. The biochemical pathways involved in increasing p53 protein levels and altering cell cycle progression following DNA damage will be studied, including potential roles of phosphorylation, ADP-ribosylation and binding of p53 to other cellular proteins. Finally, the contribution of p53 mutations to genomic instability in neoplastic cells, due to its role in this process, will be investigated.